The present invention relates to automatic optical inspection apparatus and techniques for scanning surfaces to detect defects in manufacture, particularly though not exclusively useful for the inspection of multi-layer semiconductor devices and the like, such as wafers, chips, circuit boards, etc., wherein metal conductor patterns are provided on non-metal or insulative surfaces. Current methods using reference comparison inspection have specific shortcomings that cause them to miss certain identifiable classes of defects. The new method of reference comparison of the present invention is directed to avoiding these shortcomings and therefore now enabling the reliable finding of all defects.
One method of prior reference inspection stores a correct image of the part and compares each inspected sample part to this reference image. To avoid missing all defects in the sample, however, requires very precise alignment of the sample and reference images, which demands perfect overlap. Commercial mechanical stage systems simplify cannot provide such accuracy; and even to approach that level of precision, becomes prohibitively expensive.
To eliminate this problem, the system may compare small areas, or elements, of the images. Unfortunately, this method fails due to the tradeoff between alignment accuracy and minimum defect size. As the defects grow smaller, the alignment tolerance decreases as well.
Another method of reference comparison stores a record of major features of the part, such as line ends and T intersections. This end/T-storage method also inspects by searching small areas, but it can detect small breaks that the first method cannot.
Neither of the above-described methods, furthermore, can reliably find incomplete breaks, or xe2x80x9cmousebitesxe2x80x9d.
In accordance with the new technique of the present invention, for the first time, it is believed, all defects, including those undetectable with prior inspection methods, can now be reliably and consistently detected. While also using the principle of reference comparison inspection, the invention introduces novel conditioning or processing of the reference image and conditioning or processing of the sample image, which, together with a standard design-rule inspection, now achieves a new and more reliable inspection system.
An object of the present invention, accordingly, is to provide a new and improved method of and apparatus for automatically optically inspecting for defects, electronic parts, such as wafers, chips, circuit boards and the like, comprising metal conductor and non-metal surfaces portions, and that, while using reference comparison as part of its technique, is not subject to the above-described and other limitations of prior art inspection techniques.
A further object is to provide such a novel technique wherein all manufacturing or other defects can now be reliably and consistently detected and flagged.
Still another object is to provide a novel inspection methodology more generally useful for all kinds of different-material surfaces and the like.
Other and further objects will be explained hereinafter and are more fully delineated in the appended claims.
In summary, however, from one of its important and novel aspects, the invention embraces in the optical automatic high speed scanning inspection of electronic part samples and the like comprised of conductor patterns on insulator surfaces, and wherein defects in the conductor/insulator, including very small and/or irregular shape breaks in the conductor down to the order of one pixel may be present, and are to be detected by comparison with a reference part, a method of inspection, that comprises, producing scanned images of the sample and reference parts; conditioning the reference image by shrinking it to a one-pixel thin line to serve as a skeletonized reference image; conditioning the sample image through processing it by first shrinking it from W pixels wide to a one-pixel thin line; further processing by shrinking the length of the sample-image one-pixel thin line, N pixel shrinks, while maintaining connectivity of the line pixels; and completing the processing by expanding the shrink sample line image by W pixels, thereby effecting amplified and regularized-shape break defects N pixels in length in a sample image W pixels wide; and comparing the processed sample image to the skeletonized reference line image, with an alignment tolerance of W/2, to find non-corresponding points.
Preferred and best mode embodiments and techniques are later explained in detail.